Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async queue

On 31/10/2025 01:27, Joel Jacobson wrote:
> On Fri, Oct 31, 2025, at 00:08, Joel Jacobson wrote:
>> On Thu, Oct 30, 2025, at 14:25, Heikki Linnakangas wrote:
>>> Joel, since you've been working on some optimizations in this area too,
>>> would you happen to have some suitable performance test scripts for this?
>>
>> Glad you asked. I'm actually working on a benchmark+correctness tester.
>> It's very much work-in-progress though, don't look too much at the code,
>> or your eyes will bleed.
>>
>> It's a combined benchmark + correctness tester, that verifies that only
>> the expected notifications are received on the expected connections,
>> while at the same time doing timing measurements.
>
> To run multiple pg_bench_lino processes in parallell to simulate
> concurrent workloads, I realized the randomization of the channel names
> and payloads were not random enough to avoid collissions. New version
> attached that uses real UUIDs for channel names and payloads.

Thanks! Here's a sketch for holding the bank lock across
TransactionIdDidCommit() calls. In quick testing with your test program,
I can't see any performance difference. However, I'm not quite sure what
options I should be using to stress this. My gut feeling is that it's
fine, but it'd be nice to do construct a real worst case test case to be
sure.

There are some opportunities for micro-optimizations here:

* IsListeningOn() is kind of expensive if a backend is listening
multiple channels. We really should turn that into a hash table. As the
patch stands, I'm doing the IsListeningOn() calls while holding the bank
lock, but unless we speed up IsListeningOn() in those degenerate cases,
we should perhaps call it only after copying and releasing the lock.

* If IsListeningOn() is made fast, it might make sense to call it before
TransactionIdDidCommit().

* Implement the hint bits.

I don't know how much those matter. Again, a test case would be nice.

I'll work more on performance testing next week of this, if no one else
picks that up.

- Heikki